In spectrum analyzers, incoming signals of unknown frequency are received and processed to determine their frequency, among other features. This is generally done by causing a receiver circuit to sweep or scan a frequency spectrum, across a band that includes the unknown signal, and determining the frequency of the unknown signal by display or indication on a suitable display such as a cathode ray tube calibrated in frequency.
One way which has been used is to sweep a local oscillator frequency over a desired range and to heterodyne it with the input signal. By using a fixed intermediate frequency, the local oscillator frequency value becomes an indication of the input frequency value, differing from it by the value of the intermediate frequency. The sweeping is terminated when an input signal is received which produces the fixed intermediate frequency. The frequency of the local oscillator is then determined by comparing it with a "comb" of frequencies, derived from a variable control oscillator (VCO) producing an output rich in harmonics, the set of harmonics being spaced at discrete intervals equal to the fundamental frequency F.sub.VC of the variable control oscillator. The VCO is varied in frequency until a harmonic (e.g. the N-th harmonic) of F.sub.VC differs from the local oscillator frequency by a reference frequency F.sub.R. Then the harmonic order N is determined, and the local oscillator frequency becomes determined as N.times.F.sub.VC .+-.F.sub.R. The input frequency F.sub.IN then is known as displaced from the local oscillator frequency by the fixed intermediate frequency F.sub.IF .
In such an arrangement, an ambiguity can arise, in that the local oscillator frequency may heterodyne with a comb frequency by being either above one comb frequency (which may be a harmonic N of the fundamental F.sub.VC of the VCO), or below the adjacent harmonic (e.g. N+1) of the comb. The results may be only slightly different in the two instances, and insufficiently different to permit ready distinguishing by usual techniques. In one instance, there may be a 7 megahertz disparity between the two situations, when dealing with a range of frequencies from zero up to about 2 gigahertz.
The present invention is specifically directed toward providing a simple and convenient way of resolving that difficulty, and thereby providing an enhancement of the accuracy of the frequency determination in such situations.